Tunable amplifier having a predetermined band-pass characteristic throughout its range



June 1951 c. J. VAN L(JO ETA N L TUNABLE AMPLIFI HAVING A PREDETERMINEDBAND PASS CHARACTERISTIC' THROUGHOUT ITS RANGE Filed July 15, 1946 70INPUT SIGNAL SOURCE Z INVEN TQRS. CAIZEL JAN WINLOQM H BERNARD USDOMlZWCUJ/IUBLRTUJ IZLUbL'N AGENT.

Patented June 5, 1951 rujdvieus.v Ana ram HAVING A Patina-- TERMIN'ED'BAND-PASS- CHARACTERISTIC rnnouonoo'r rrs RANGE Carel Jan vanLoon andBernardus Domin'icus Hubertus; Tellegen, Eindhoven, Netherlands;assignors to'Hartford National Bank and Trust Company, Hartford, Conn,as trustee Application July 15, 1946, Serial No. 683,708 In theNetherlands November 11, 1942 Section 1, Public Law 690, August 8, 1946Patent expires November 11, 1962 2 Claims. 1.

The United StatesPatent No. 2,352,956 issued July 4, 1944, relates to acircuit arrangement for the transmission of high-frequency electricaloscillations, which comprises at least one controlled discharge tube andis' equipped with an input resonant circuit which is" tuned to thefrequency of the oscillations to be transmitted, and in which the inputresonant circuit proportioned in such manner that the damping broughtabout by the losses of the circuit of its own, plus the natural inputdamping of the discharge tube and the damping exerted on the circuit bypreceding parts of the arrangement, is smaller than is necessary inconjunction with the width of the frequency band to be transmitted, therequired additional damping being brought about by withdrawing energyfrom the circuit by means of a negative backcoupling, the backcouplingcurrent or voltage being taken from the circuit(s) of one or morecurrent carrying electrode(s) which are chosen in such manner that thenoise current in the output circuit is reduced at the same time onaccount of the negative backcoupling.

In the circuit arrangement according to the principal patent the valueof the negative backcoupling generally depends on the frequency to whichthe input resonant circuit is tuned. Consequently a variation in tuninginvolves a variation of the degree of the negative backcoupling and, asa result thereof, of the selectivity of the input resonant circuit.

In the circuit arrangement according to the present invention thisdisadvantage is obviated by providing means as a result of which thedamping of the input resonant circuit remains at least substantiallyconstant in tuning over a wide frequency range. To achieve this constantdamping the negative backcoupling is preferably so realised as to bemixed inductive-capacitative. Thus, for instance, the input resonantcircuit may be connected on the one hand to the anode and on the otherhand to the cathode of the discharge tube, the control-grid beingconnected to a tapping of the capacity of the circuit and thecontrol-grid circuit or the anode circuit including an inductance coilwhich is inductively coupled with the resonant circuit.

The same result is obtained when interchan ing the connections of theanode and the control gridto the resonant circuit. In' the manner statedabove the capacitative backcoupling has a damping effect on the resonantcircuit. Oi course, the inductive backcoupling should also be realisedin such manner as to damp the resonant circuit.

The invention will be more fully explained by reference to theaccompanying drawing representing, by way of example, one embodimentthereof.

The sole figure represents an amplifying circuit arrangement, in whichthe resonant circuit I is tuned to the frequency of the electricaloscillations to be transmitted. For simplicity the direct voltagesupplies have been omitted in the drawing.

The resonant circuit consists of a variable condenser 2, a fixedcondenser 3 and an inductance coil 4, the latter being coupled with aninductance coil 5 interposed in the cathode lead of the discharge tube6. The control-grid of the discharge tube 6 is connected to the top ofthe circuit I. To the other end of this circuit is connected the anodeof the discharge tube and this across a resistance 1 from which theamplified voltage may be taken. As an alternative a resonant circuittuned to the signal frequency may, of course, be substituted for theresistance in the present case. The coupling between coils 4 and 5 is sochosen as to damp the circuit I, whilst the capacitative couplingbetween anodeand gridcircuit, which is established by the condenser 3,also has a damping effect. In this way a resulting damping of thecircuit I is obtained, which upon variation of the variable condenser 2remains constant or substantially constant over a wide frequency range.Consequently the ratio between the loss resistance and the inductance ofthe circuit, which is determinative for the selectivity, has at leastapproximately the same value for all frequencies of the tuning range, sothat the selectivity does not depend on the tuning frequency.

What we claim is:

1. A circuit arrangement for the transmission of electrical oscillationscomprising an electron discharge device having a cathode, a grid and ananode, a resonant input circuit tunable within a prescribed frequencyrange and provided with a capacitor and an inductor,

an output impedance, and a degenerative network formed by a coil inseries with a condenser, one end of said output impedance beingconnected to said anode, the other end of said output impedance beingconnected through said condenser and said coil in the order named tosaid cathode, one end of said inductor being connected to said grid andthe other end thereof being connected to the junction of said coil andsaid condenser, one end of said capacitor being connected to said gridand the other end thereof being connected to said other end of saidimpedance, said coil and said inductor being mutually coupled, said coilhaving developed thereacross a noise voltage due to cathode noisecurrent flow therein to establish a noise voltage across said inputcircuit in phase opposition to the noise current flow in said coilthereby to increase the damping of said input circuit without decreasingthe signal to noise ratio, said condenser further damping said inputcircuit to provide a predetermined bandpass characteristic theretowithin said prescribed range.

2. A circuit arrangement for the transmission of electrical oscillationscomprising an electron discharge device having a cathode, a grid and ananode, a resonant input circuit tunable within a prescribed frequencyrange and provided with a capacitor and an inductor, an outputimpedance, and a degenerative network formed by a coil in series with acondenser, one end of said output impedance being connected to saidanode, the other end of said output impedance being connected throughsaid condenser and said coil in the order named to said cathode, one endof said inductor being connected to said grid and the other end thereofbeing connected to the junction of said coil and said condenser, one endof said capacitor being connected to said grid and the other end thereofbeing connected to said other end of said impedance, said coil and saidinductor being mutually coupled, said coil having minimum resistance andhaving developed thereacross a noise voltage due to cathode noisecurrent flow therein which is applied through the interelec-o REFERENCESCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,243,401 Sturley r May 27, 19412,293,262 Koch Aug. 18, 1942' 2,315,040 Bode Mar. 30, 1943 2,352,956

Strutt et a1. July 4, 1944

